WASHINGTON (AP) — Air traffic in the U.S. could “slow to a trickle” if the federal government shutdown lingers into the busy Thanksgiving travel holiday season, Transportation Secretary Sean Duffy warned Sunday.

The Federal Aviation Administration last week ordered flight cuts at the nation’s busiest airports as some air traffic controllers, who have gone unpaid for nearly a month, have stopped showing up for work.

The reductions started at 4% and will increase to 10% by Nov. 14, impacting all commercial airlines from 6 a.m. to 10 p.m. local time.

On Sunday, more than 1,700 flights were canceled by early afternoon, with nearly 5,000 flights delayed, according to FlightAware. That follows over 1,500 cancellations on Saturday.

Hartsfield-Jackson International Airport in Atlanta led with the most cancellations Sunday, totaling 173, followed by Newark Liberty International Airport with 115.

The FAA indicated that staffing shortages at Newark and LaGuardia resulted in average departure delays of around 75 minutes.

Detroit Metropolitan Wayne County Airport showed a notable decrease in activity Sunday morning, experiencing low passenger traffic as cancellations affected both departures and arrivals.

Duffy noted that potential additional cuts could reach 20% if air traffic controllers remain unpaid. “More controllers aren’t coming to work day by day, the further they go without a paycheck,” he remarked during an interview.

As Thanksgiving approaches, Duffy cautioned travelers about the upcoming chaos, predicting a significant slowdown in air travel as the holiday draws near. “You’re going to have massive disruption. I think a lot of angry Americans,” he stated.

Staffing problems have already disrupted over 4 million passengers from Oct. 1 to Nov. 7, according to Airlines for America, as delays related to air traffic control staffing exceeded 3,000 hours last Saturday.